dev-lang/ldc2 lto support

[the-horo]

Emerging ldc2 yields:

QA: other
QA Notice: Package triggers severe warnings which indicate that it
           may exhibit random runtime failures.
/var/tmp/portage/dev-lang/ldc2-1.36.0/work/ldc-1.36.0-src/gen/abi/x86-64.cpp:191:45: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
/var/tmp/portage/dev-lang/ldc2-1.36.0/work/ldc-1.36.0-src/gen/asm-x86.h:3869:64: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
/var/tmp/portage/dev-lang/ldc2-1.36.0/work/ldc-1.36.0-src/gen/asm-x86.h:3872:64: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
/var/tmp/portage/dev-lang/ldc2-1.36.0/work/ldc-1.36.0-src/gen/asm-x86.h:3869:64: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
/var/tmp/portage/dev-lang/ldc2-1.36.0/work/ldc-1.36.0-src/gen/asm-x86.h:3872:64: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
Please do not file a Gentoo bug and instead report the above QA
issues directly to the upstream developers of this software.
Homepage: https://github.com/ldc-developers/ldc

This warnings are problematic as they indicate runtime issues with lto as detailed in https://wiki.gentoo.org/wiki/Project:Toolchain/LTO#Signs_a_package_is_unsafe.

There is also no documentation describing how to turn on lto for D packages.

[the-horo]

This issue has been hit at https://bugs.gentoo.org/922590